A configuration in which a plurality of antenna units having some base station functions are physically extended and which is used as a distributed unit in a wireless communication system particularly, a mobile communication system, has been studied. Here, the antenna unit refers to a unit having a transmission interface, a wireless transceiver, and antennas. As a central unit, a base unit obtained by extending a distributed unit has a function of controlling a plurality of distributed units. In a communication scheme of performing communication between a central unit and a distributed unit, two system configurations called full centralization and partial centralization in which a function distribution between the central unit and the distributed unit is different are being studied.
As illustrated in FIG. 1, full centralization has a system configuration in which a central unit 81 has functions of a data link layer or higher and a physical layer from which antenna units are excluded and distributed units 82 having only an antenna unit are extended.
The other system is referred to as partial centralization which includes a system configuration in which functions of a data link layer or higher are installed in a central unit 91 and distributed units 92 having a physical layer function including an antenna unit are extended as illustrated in FIG. 2 (e.g., see Non-Patent Document 1).
While the physical layer is controlled within the central unit 81 in full centralization as illustrated in FIG. 1, the central unit 91 and the distributed unit 92 need to exchange control information of the physical layer in partial centralization as illustrated in FIG. 2. Currently, full centralization is a more widely used system configuration. In this configuration, a communication scheme using digital radio over fiber (RoF) technology represented by a common public radio interface (CPRI) (e.g., see Non-Patent Document 2) is used as a communication scheme between the central unit 81 and the distributed unit 82.
On the other hand, as illustrated in FIG. 3, one base station 94 connected to a core network 93 covers an area called a cell in a mobile communication system. In such a mobile communication system, a phenomenon in which a radio signal transmitted from a desired base station 94 and a radio signal transmitted from an adjacent base station interfere with each other when a terminal device 95 which is a mobile station, reaches an area of an edge of the cell and a transmission speed between the base station 94 and the terminal device 95 is remarkably degraded is problematic.
Coordinated multi-point transmission/reception (CoMP) technology (e.g., see Non-Patent Document 3) is being studied as a means for solving a problem of inter-cell signal interference. As illustrated in FIG. 3 and FIG. 4, in this CoMP technology, adjacent base stations 94 connected to the core network 93 (FIG. 3), the central unit 81 or 91 and the distributed units 82 or 92 connected to the core network 93, or the distributed units 82 or 92 (FIG. 4) communicate with each other in cooperation with the terminal device 95, which is a mobile station located at a cell edge. Also, there is joint transmission (JT) for transmitting signals from different base stations 94, the central unit 81 or 91, and the distributed unit 82 or 92, or the different distributed units 82 and 92 at the same time/frequency as one technique for implementing CoMP in downlink. In JT, a method of transmitting signals to be transmitted from different cells after a precoding process is performed in advance is called coherent JT.